The need for environmentally acceptable energy resources remains a primary concern to industry and government, and methods for desulfurizing high sulfur coal have been one primary area of interest in this regard. While coal resources are plentiful, the quality thereof with regard to nonpolluting combustability varies from region to region. Coal quality, from the standpoint of air quality standards, is predicted principally upon the sulfur and nitrogen content thereof. There are generally three forms of sulfur in coal: (a) organic sulfur including organic sulfates, (b) pyritic sulfur, FeS.sub.2, and (c) sulfate. The majority of the sulfur in most coal is in the form of iron pyrite.
The impetus for coal desulfurization can be traced to the acid rain issue and The Clean Air Act of 1970. Coal combustion plays a part in the deliberations concerning both issues. Many experts believe acidic precipitation is due to coal combustion, and much of the coal in the United States will stay in the ground until acid rain is better defined or until an efficient method for desulfurizing high sulfur coal is developed.
Most of the research into coal desulfurization has focused on pyritic sulfur, since this generally represents at least half of the sulfur in coal.
Numerous methods for desulfurizing coal have been attempted. These methods include physical separation techniques, chemical processes, and bacterial oxidation. Physical separation processes have not generally been successful, due to the smallness of particle size necessary for pyrite liberation. Chemical processes using a variety of solvents including quinoline, toluene, petroleum ether, and household bleach, have met with success in the laboratory, but these methods have not proved to be economically and technically acceptable on an industrial scale.
Thiophilic bacteria, such as thiobacillus ferrooxidans and sulfolobus acidocaldarius, have been a major area of interest due to their ability to dissolve the pyrite from the coal by oxidizing pyritic sulfur to soluble sulfates, which may then be separated from the coal. This method consists of the total oxidation of the pyrite which may take from three to fifteen days, and has not yet proved to be economically and technically acceptable on a commercial scale.
Physical-chemical separation techniques such as oil agglomeration and froth flotation have been attempted, the latter being used commercially. These techniques for purifying coal are based generally on the relative hydrophobicity of the coal as compared to the impurities to be separated. Unfortunately, the surface chemical behavior of coal and pyrite are quite similar and therefor their relative hydrophobicity is not sufficiently different, especially under commercial separation process conditions, to provide commercially acceptable separation of pyrite from high sulfur coal. For example, in the past others have treated coal slurry with T. ferrooxidans bacteria for fifteen to sixty minutes and then attempted to remove the pyrite from the coal slurry by an oil agglomeration technique. The results of this technique have generally been inadequate because relatively large quantities of oil must be used in the process thus making the process unacceptably expensive. In addition, the mixing of the bacteria with oil during the agglomeration process contaminated the bacterial solution and therefor limited the number of times the bacterial solution could be reused. In addition, relatively long treatment times were required which resulted only in the removal of relatively smaller percentages of pyrite.
Froth flotation is a conventional process which has been successfully used in a variety of mineral separation processes and depends upon the difference in the surface properties of the particles which constitute the slurry. In the froth flotation process, the material is ground to liberate particles of the materials to be separated. Flotation reagents such as a collector and a frother are mixed with the slurry and the slurry is then simultaneously aerated and stirred. Collector reagent is adsorbed selectively on the surfaces of the hydrophobic minerals and the hydrophobic minerals then become attached to the ascending air bubbles where they are collected on the slurry's surface in the form of froth. The hydrophillic mineral is depressed and sinks to the bottom of the flotation cell which results in the physical separation of the minerals.
Although this froth floatation process has been widely used for coal desulfurization, results have not been satisfactory because of the incomplete separation of pyrite from coal. The separation has been incomplete in part because the surface properties of both coal and pyrite particles have relatively similar hydrophobicity and thus both tend to float readily on the aerated slurry resulting in incomplete and inadequate separation.